Pulse generating circuits



1950 w. s. GRAFFBAKER ET AL 2,923,841

PULSE GENERATING CIRCUITS Filed Nov. 22, 1955 F|G.l.

N FLUX DENSITY B l o D B C MAGNETIZING FORCE H.

INVENTORSI WILLIAM S.GRAFF-BAKER,

DAVID J. STOREY,

THEIR AGENT.

United States Patent PULSE GENERATING CIRCUITS William SebastianGralf-Baker, Dunchurch, and David John Storey, Rugby, England, assignorsto The British Thomson-Houston Company Limited,a British company 1Application November 22, 1955, Serial No. 548,458 Claims. 1 or. 307-406)This invention relates to pulse generating circuits, and moreparticulary to generating circuits of the type in which energy is storedin a pulse forming network and is periodically discharged into a load byoperation of a switching device.

It is usually found that in such a circuit a reverse voltage appearsacross the switching device immediately following the discharge whichmay be caused by unavoidable mismatch between the load andthe pulseforming network, pulse transformer magnetizing current, and straycapacitance associated with the loadand the transformer windings, and isaccentuated by the action of aj load such has a non-linearvoltage-current with the discharge device have beenused to minimize thenegative anode voltage, but theseare not completely effective and diodessuitable for use with pulse generators of present output power have notyet been developed. In addition, this circuit is not applicable to pulsecircuits employingan alternator charging system.

One object of the present invention is to prevent the rise of negativeanode voltage across a gas discharge switching device. 1.

Another object of this invention is to delay the application of suchnegative anode voltage across a gas discharge switching deviceuntil'thedischarge space is deionized sufiiciently to avoid serioussputtering. e

A further object of thisinvention is to provide signal control means forpulse circuits.

A circuit constructed in accordance with one embodiment of the presentinvention comprises an energy storage circuit, a discharge circuit fordischarging the energy stored in the storage circuit into a loadcircuit, and a saturable inductor connectedin series with said dischargecircuit. L

In accordance with a modification of the invention a saturable reactoris connected in the discharge circuit. "This circuit will providecontrol of the rate of rise of pulse voltage, pre-pulse ionizationof thegas discharge switching device, and provides a pre-pulse for triggeringmonitoring equipmentin addition to the main object of preventingapplication of negative anode voltage to gas discharge switching device.

The features of our invention which we believe to be novel are setforthwith particularity in the appended further" objects and advantagesthereof, may best be un- .derstood by reference to the tollowingdescription taken ing there until the fiux in the core is built up2,923,841 Patented Feb. 2, 1960 in connection with the accompanyingdrawings in which:

Figure 1 illustrates hysteresis characteristic of asaturable inductorsuitable for use with this invention.

Figure 2 is a schematic diagram of a circuit constructed in accordancewith the present invention.

Figure 3 is a schematic diagram of a modification of the circuit shownin Figure 2. v

Figure 4 is a schematic diagram of another modification of the circuitshown in Figure 2. In Figure 1 of the drawings the charactertistics of asaturable inductor suitable for use with this invention is shown. Thesaturable inductor is a coil wound on a core having a Bl-l loop as shownin Figure 1. When the core is in mignetic saturation, that is 3-1 theportions of the loop, BCD and EFG, the reactor has a small inductance.In the unsaturated state, represented by a portion of GB and DE, it hasa high inductance common to iron core inductors.

- In Figure 2 of the drawings one embodiment of the pulse generatingcircuit according to the present invention is shown to include a source10 for charging the circuit, having one of its terminals connectedthrough an energy storage device or pulse forming network .11 and asaturable inductor 12 to theterminal 13. Pulse forming network 11 in oneembodiment comprises a plurality of series inductors and a plurality ofshunt capacitors. The remaining terminal of source 10 is connecteddirectly to terminal 14. The utilization circuit such-as the primarywinding of a load coupling transformer is connected across terminals 13and 14. The secondary 16 of the load transformer is connected seriallywith the load 17. A switching device, such as a thyratron 18, isconnected in shunt across the source. The capacitance 19 represents thestray capacitanceacross the switching device 18. The capacitanceincludes the tube inter-electrode capacitance and the stray capacitanceof the pulse forming network.

The saturable inductor 12 is dimensioned so that when the core is in theunsaturated state it has a relatively high inductance and its impedanceis much greater than that of bot'1 the load referred to the primary 15of the coupling transformer and the stray capacitance 19 across theswitching device. When the core is in magnetic saturation the inductor12 has such as small inductance that its impedance in the circuit isnegligible. The operation of this invention may be best understoodbyreference to Figure 1 and 2 together. 7

If the pulse forming network 11 of Figure 2 is initial- 1y considered asfully charged and the saturable inductor 12 is considered to be at pointA on its hysteresis curve as illustrated by Figure 1, the operation ofthe circuit for the rest of the cycle is as follows:

When the switching device such as the thyratron 18 is triggered, theenergy stored in the pulse forming network 11 will flow through thecircuit, including the switching device, pulse forming network,saturableinductor'and the utilization circuit. The small current flowingthrough the windings of the saturable core inductor will carry the coresaturation to the unsaturated state illustrated as point G in Figure l.

When the core or the saturable core inductor is 'unsaturated thesaturable inductor is the highest impedance element in the circuit.Therefore, almost the full charging voltage appears across the saturableinductor, remainto saturation, illustrated as point B in Figure l.

When the core of the inductor saturates, the inductor assumes a very lowvalue and effectively connects the pulse forming network directly to theutilization circuit. The energy stored in the pulse forming network isdischarged into the utilization circuit. In so doing, the core Meagan isforced up-to point C by the discharge "currenhforthe duration of thepulse. 3

I, At the end of the pulse dischargeperiod, any residual reverse "voltzig' on thetrafisfOr-nier'windings 'of 'theifplllse 'farmingnerwprkw'ireausetheeurrntwne inaf direction-opp'o's -it othat of the current fi wdiiringthef'rnain p'ul 'se. -"lA very small current flow [car iesthecore back [to the aturatd "state; pointD in Figure 1. fWhe'n the breashamed; the 'inducthr' isa'g i e e mathe f the ire eliafge 'on thtransformer -winding capacitance, the time interval to reach saturationmay be flong' enough for. the stared e ''i gy 't'o'be dissipated in" thetransformercore 6r"other'-'-'resistive s'huntcircuitelements. I Inthisevent, i'evrs e "voltage is completelyprevented from reaching theswitc'hing 'device, for 'examplefthe anode of a thyr'atron.

- if, hewevnaeew load mismatchor transformer magv tiiid'g" i iii'gfi'twor whiehcahnothe dissipated in the" resistance, Pith reactowillstilliOperate" efiiciently lby withholding i'legititre-" X76;ltage"from the' thyratron' anode when such-a sviiitchingdevice is used,untilthe thyratron has sufiiciently as mzed-toavoid serious sputtering.Alargeresidual arge'wiu -can uie 'core'into saturation, *such asilluspoim Fin Figure "1 Assbdn as this reverse ge-has died-out, "theco're will return t'o point A empering-meansenemaymg the circuit for thenext pul's'e; A, v I Al'tliduh it-is'hsuall'y possible "remake theunsaturated impedance on the saturable core inductormuchlarger than th'eimpedance'of the stray c'apacitance'19, it may always bepractical. 'Inthis"event,"as'will'be obvious "siefskilled in the art;'a' capacitorrnaybe' connected 111 shunt withf'the switching device to'augment the strayt: pacitance across 'the pulse network. 7 p v n alteriiate positionin'the' circuit{fol-{placement of the 's'at'urab'le'core inductor-is byconnectionfin'seriesiwith h r" witching device. If a gas thyratron'is11sed, the inductor willb'e "connected 7 to the anode.

Operation is essentially the'jsa'me as described-above "eJicept'thatV-there is no network charging currentto drive '-the core to pointF- in Figure 11. The' bottom end of the :i hysteresis loop vvouldtake'aform such asthat represented by the dotted line in Figu're l.JOE-"course, -with such a connection: the stray capacitanceacross thes'witeh zwwo'uld-not include the stray capacitance-of the'puls'e form-;-ing-,network. Thestray capacitancej should beincreased abyiairesistance and capacitancemetwork connected in {shunt; withthe-switching device 18, Such design changes I 's'hould be readilyapparent to those skilled ai'ntthe art.

7 Although the main objectof our invention is to prevent negativevoltage appearing at-the switching device,partic'ularly the anode of agas discharge tube,--and--thereby causing damage to the. switchingdevice, there are in- ,h remreamres which area dvantageous-inspecialized applieations. t p n v M It'is evident from lthe foregoingdescription thatthe U f Tflow of pulse currentt is controlledby the coreof "asa-t'ur'ablereactorlgoing in the saturation. There fore the rate ofbuildup of current is controlled not only 'liytlie characteristic of thenetwork, but alsoby the itransi'tion from" thefunsaturated'to thesaturated state of theeore" of the "saturiable inductor. The latter is'de- 'f ctdjb e curvatu're otthe'liysteresisldop near "oint i B -add' isa fttnetioh ofthe"type"afidniateriaf'of the core.

e'nt' a'" reverse" charge is left on the pulse fo'rmmagnetizing currentis" of"the"orderof 2% "of'the J pulse current, the magnetizing currentwill serve .to partially ionize' a gas tube u'se'd 'as the switchingdevice, thereby aiding in theformation of a high pulse current are. This15 is of particulanirfiportance"whn anigni'tron is used for theswitching device.

gas discharge tube is used as a switching devi'ce "fall ofvoltagemay,therefore, be'iconvenientlyused'as a ire-pulse'for 'controlling'i dicatiflg O 'Y'rn stan 2 and capacitance 1 p v, Ittie'swit'chi'iig'devieezz. fInthe usual'i'gnit rifi'rin'g ci toffthis'type the output current" is limited b'yfthe inductancejof an inductorcircuitelement. ltf'isfinli'ereht insuchf'an arrangemeat that a"voltageftif nearly" the sai'fiefrna'gnitu'de as the for- "wardvoltagebut of opposite'jpolarity'appears a'fo'ssthe switchfollowing thedischarge. f v In ford'erfto 'fprof" ftlie' switchingidevicej it hasbeen usuarw shuntthe"'switching 'devi hy means bfa series resistor andcapacitor Such a circuit cdmb'ination is effective 'on1y"at "1o w filfi'rifcies inikeepin'gthe rate -of riseofthevdlfiagehfo 'site'p'dliirit'ydh'w n to ape'rniiss'ible "value. "To be ve 'fl'in "a circuit fdsigned to fire anignitor in the 1m off e'ordrfof micrseconds,-theshunt'iesis'tariceand acitinceassilhie Values which would interferewith theinbi mal'ichairgingicycle of a "storageeapa'citdp I,

p In' aecordariee 'this-embddiirient'bff ourilinvefition, "asaturable"reac"tor"is .use'dasl'f'acirciiit element. The faett'jr'i'sdimensio Yso' hatE't he saturated inducement" the saturahleco're i actbr' is "the value of the inductan cerequiredto limit .theoutput currentto tlreflbad. When 'tlie c oie isunsaturatdihoweyer, the

induc'tan cejisdimdiisibhdio befthe highest impedance 55 inducta ce thin11' I i A V shunt'ed y efie'ctof the shu' v made 'negligible"b 'causeTiif saturable core inductor 21. ,7 t v 7 I The"jnode'ofiopfatidnfo thiscir uit asjfollowsz During the fcharging cyclwhen 'capaci't'or 20ischarged '10,?tlie saturable eer e inductance z l is in t tlfby- PQQPPAr u e-P 2? 1 tri ered cu renestar 1e core inductor'carryingthe coreto the saturated state represented by point B, Figure 1. w en the fluxin the core reaches saturation the across it until the core has beendriven to saturation in the opposite direction. The inductance isdesigned to make this time interval long enough to allow a switch 22,when a gas discharge device isitsed, to deionize sufficiently to avoidanode sputtering by-application of a large negative voltage tothe anode.I

In a circuit of this type, the full charging voltage of the networkappears across the reactor during the time that the core is building upto saturation. In practice, the time taken for this build up of fiux maybe of the order of a fraction of a microsecond. Such a'rate' ofchange offlux will cause considerableeddy current loss in the core of theinductor which necessitates, particularly at'highpulse, repetitionrates, careful design of the Secondary featuresof this circuit are acontrol of the rate of rise of pulse voltage by inductordesign.Pre-pulse ionization of the switching device .by the small vision of apre-pulse for triggering monitoring equipment. The provision forpre-pulse is a direct consequence of the time interval betweentriggering the switch and discharge of the network. This feature is,therefore, obtained at the expense of appreciable loss in the reactorcore. In some circuits the provision of a pre-pulse without time jitteris not required. In this event, a significant reduction of core lossesmay be achieved by the modification shown in Figure 4.

In Figure 4 the circuit is shown to include elements similar to thoseshown in Figure 2 and elements are identically numbered. The circuit isshown to include a source 10 connected through a pulse form-ing network11 to a terminal of a primary 15 of an output transformer. The otherterminal of source 10 is connected directly to the other terminal on theoutput transformer primary. A serially connected saturable inductor 30and a switching device 18, such as a thyratron, is connected in shuntacross the terminals of source 10. A saturable core inductor has abiasing winding mounted on its core. The biasing winding 31 is energizedthrough a blocking choke 32 from a source of direct voltage 33. As inFigure 2, the capacitor 19 represents stray capacitance across theswitching device 18.

If the pulse forming network 11 is fully charged and the bias winding 31of the saturable core inductor is arranged to bias the core to thedegree represented by point B in Figure l, the subsequent operation isthe following: When the switch 18 is triggered, the inductor, already inthe saturated state, offers low impedance to the flow of current and thepulse form-ing network immediately discharges through the load. Thedischarge current through the saturable core reactor will carry the coreflux density up to point C in Figure 1. After discharge of main pulse,any residual reverse voltage will carry the core to point D when theinductor becomes unsaturated, becomes the highest impedance element inthe circuit branch, and, therefore, drops the reverse voltage across itswindings. After the current due to the reverse voltage ceases, thepolarizing current through the auxiliary winding 31 Will restore thecore condition to point B of Figure 1, whereupon the circuit is re-setfor a second pulse.

Since the interpulse period is available for swinging the flux frompoint G to pointB, the rate of flux change which maybe controlled by thepolarizing'supply voltage can be made relatively small withcorrespondingly small eddy' current'los's in the core. The majorproportion jof the loss in this case'occurs while the reverse voltage ischanging flux from'point D 'to'point F, but this is not a large loss asthe change is relatively slow, taking place over a period of severalmicroseconds. In this manner, the heating of the core of the saturableinductor is reduced and special cooling design is obviated.

A similar mode of operation of the circuit shown in Figure 4 is obtainedif the'reactor is connected between the pulse forming network 11 and theprimary trans former 15. The effect of the polarizing current in thistype 'of connection will be opposed by the network charging currentuntil the charging current has fallen to a 125 core to limit eddycurrent losses ina manner well known to the art and may necessitatespecial cooling means.

magnetizing current flowing before saturation, and prosmall" value, thusleaving a small proportion of the interpulse period available for thepolarizingsupply to restore the core condition to the saturated'state,represented by point'B in Figure 1.

While particular embodiments of the present invention have been shownand described, itwill be obvious to those skilled in the art thatchanges and modifications may he made without departing from theinvention in its broader aspects, and, therefore the aim in the appendedclaims is to cover 'all such changes and modifications as fallwithin'the true spirit and scope of the invention. -What we claim as newand desire to secure by Letters Patent of the United States is:

1. In combination, an'ignitron firing circuit, a source ofdirectpotential having first and second terminals, 11* first capacitor,said capacitor connected across said source, an ignitron having anignitor terminal and a ground terminal, a saturable inductor, a gaseousdischarge device, said inductor and device serially connected betweenthe ignitor of said ignitron and said first terminal of said source,said ground terminal being directly connected to said second terminal ofsaid source, and a resister and second capacitor, said resistor andsecond capacitor being serially connected across said gaseous dischargemeans.

2. In combination, in an ignitron firing circuit, a source of directvoltage, energy storage means, said energy storage means comprising alinear capacitance connected in shunt with said voltage source, anignitron having an ignitor, switch means and saturable reactor meanscoupling said ignitor to said energy storage circuit, said switch meansadapted to discharge said energy storage circuit into said ignitor, saidreactor means adapted to allow discharge of the energy storage circuitinto said ignitor and to prevent flow of energy for the ignitor intosaid storage circuit immediately following said discharge.

3. In combination, a source of direct voltage, an energy storage circuitcoupled to said source, a load circuit, said load circuit coupled tosaid energy storage circuit, a saturable core inductor, and a gaseousdischarge device, said inductor and gaseous discharge device beingserially connected across said source, said saturable inductor havingbiasing windings, said windings being adapted to adjust the initialmagnetization of the core of said saturable core inductor.

4. In combination, a source of direct voltage, an energy storage circuitcoupled thereto, a load circuit, a saturable core inductor, saidinductor coupling said load circuit to said energy storage circuit, andswitch means connected across said source, said means comprising athyratron.

5. In combination, a source of undulating potential, an energy storagecircuit coupled to said source, said energy storage circuit comprisingcapacitance, a controlled circuit, said controlled circuit having afirst and second terminal, a saturable reactor and a gaseous dischargedevice serially connected between said first terminal and said energystorage circuit, said gaseous discharge device being adapted to couplesaid first terminal to said energy storage circuit when the energy inthe storage '7 cirfi it reaches apredetei-jmined value, -said, saturablereactor being adapted to saturate and become a; low value of inductanceduringdischarge ofthe storage cir; cuit and a high value of inductanceatthe end of -s'aid discharge,- and fmeans coupling said second terminalto said storage circuit.

6. In combination, a. source of electric' energy, an, energy storagecircuitcoupled to said source, a ,loadcircuit, a saturable'reactor,switch means recurrently rendered operable to ,dischargeenergy into saidload circuit from said energy storagecircuit through said reactor whenthe energy stored in'said storage circuitreaches a predetermined value,said saturable reactor being responsive to operation of said switchmeansto saturate and become a low value of impedance during discharge of thestorage circuit and a high valueof impedance at the completion ofsaiddischarge..

7. Incombination, a, source of electric energy, an

energy storage circuit coupled to said source, anloadcircnit, asaturable reactor, meansrecurrently rendered operable to dischargeenergy into said load from said energy storage circuitthrough saidreactor, said saturablereactor being so dimensioned with respectto saidsource, energy storage circuit, andload circuit asto saturate. and becone a lowsvalue of impedance in resppns qtosaid dishar e anda-hig r l e fp anc wth n. aid sour e; energy tq a' ecir u and o d c rcuitiuresppu tot e completion of said discharge. 7

8-; rlnh qmbiua n, outc' otekctflc-eufism-anen y; s ora e irc it couped: o :said o rccr unoutpnt meager;

\ i 'f lit. safitblelteacttlhmeans-gcoupling saidioutput circuit ;tosaid Leher'gy storage} circuit, means reciirrently ren: deredfoperableto discharge said; storage circuit when the energy stored in said,storage circuit reaches a desirable value, said saturablereactor-meansbeing responsive ,to

said discharge .tosaturateandbecome-a-low. value of impedance-during,said, discharge and "a high value of impedanceat the completion of saiddischarge.

, 9, In combinatioma source of electric energy, an,ene'rg'y vstora ge'circuit, a load circuit, a saturable reactor, said source,[storagecircuit, load circuit and saturablc reactor being. seriallyconnected,and switch means for discharging said ener y storage network,saidv switch meansv being connected-in jshunt with saidisource.

10; In combination a sourceoff electric energy, can en'ergyst'oragecircuit connectediher'et'm-a loadcircuit a saturable coreinductorconnectingsaid:v load circuitto saidstoragecircuit, and a gseous.dischargedevice connected, across said source #7 for,dischargingsaid storage circuit. 1 i

References Clted'finjth'e file ofithis patent UNIT D. sIA'rEsPATENrs.

2,436,395 Manley Feb'.24,1948

